Comment 6 for bug 1516358

The described behaviour was introduced with the fix of bug #693337. (Nowadays the list in question is provided by the accountsservice package via the /usr/share/language-tools/language-options script, which Tim mentioned.)

Selecting language is equivalent to picking the translation of choice, so the list of languages in a GUI represents available translations. Since there is no separate Belgium translation into Dutch, it would make no sense to present more than one Dutch option in the language list. Consequently, Peter, nl_NL is good enough.

I'd consider this bug invalid.